Secondary batteries have also attracted considerable attention as an energy source for electric vehicles, hybrid electric vehicles, etc., which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
In addition, technology related to power storage apparatuses that store electric power and stably supplies the stored electric power to a power system when needed has been developed. The power storage apparatuses are apparatuses that store electric power when power demand is low and supply the stored electric power in case of overload or emergency. The power storage apparatuses provide the effect of improving quality of electric power and energy efficiency. In particular, a market for household power storage apparatuses and middle-sized industrial or commercial power storage apparatuses has been rapidly expanding as the power storage apparatuses are related to smart grid technology.
As a result, kinds of applications using the secondary batteries are being increased owing to advantages of the secondary batteries, and hereafter the secondary batteries are expected to be applied to more applications and products than now.
As applications and products, to which the secondary batteries are applicable, are increased, kinds of batteries are also increased such that the batteries can provide powers and capacities corresponding to the various applications and products. In addition, there is a strong need to reduce the size and weight of the batteries applied to the corresponding applications and products.
For example, small-sized mobile devices, such as mobile phones, personal digital assistants (PDA), digital cameras, and laptop computers, use one or several small-sized, lightweight battery cells for each device according to the reduction in size and weight of the corresponding products. On the other hand, middle or large-sized devices, such as electric vehicles, hybrid electric vehicles, and power storage apparatuses, use a middle or large-sized battery module or battery pack having a plurality of battery cells electrically connected with each other because high output and large capacity are necessary for the middle or large-sized devices. The size and weight of the battery module is directly related to an accommodation space and power of the corresponding middle or large-sized device. For this reason, manufacturers are trying to manufacture small-sized, lightweight battery modules.
Meanwhile, a larger amount of heat is generated from such a high-output, large-capacity secondary battery during charge and discharge of the secondary battery. If heat generated from unit cells of the battery during charge and discharge of the unit cells is not effectively removed from the unit cells, the heat accumulates in the unit cells with the result that deterioration of the unit cells is caused. In addition, if some of the unit cells are overheated due to various causes, the battery may catch fire or explode. For this reason, a cooling system is indispensable for a middle or large-sized battery pack having high output and large capacity
The middle or large-sized battery pack is generally cooled by the flow of a coolant. For example, a coolant flow cooling system that performs cooling through the flow of a coolant, such as air, between unit batteries or battery modules of the battery pack using a cooling fan. However, this coolant flow cooling system has several problems.
First, a temperature deviation between unit batteries is very large. The battery pack includes a plurality of unit batteries. In a state in which each of the unit batteries is in an optimal state of operation, the battery pack may also be in an optimal state of operation. Consequently, such a large temperature deviation between the unit batteries may accelerate deterioration of the batteries and make it difficult to optimize the state of operation of the battery pack.
Second, the conventional cooling system causes the increase in size of the battery pack. For example, the large-sized battery pack may not be applicable to electric vehicles (EV) or hybrid electric vehicles (HEV) since the size of the battery pack that can be mounted in the electric vehicles or the hybrid electric vehicles is limited.
FIG. 1 is a typical view showing a conventional representative battery pack cooling system.
A battery pack cooling system 10 includes a battery pack 11 including a plurality of batteries, a coolant introduction part 12 mounted at the lower part of the battery pack 11, and a coolant discharge part 13 mounted at the upper part of the battery pack 11. The battery pack 11 includes a plurality of battery groups 14 electrically connected to each other. Each of the battery groups 14 includes a plurality of unit batteries 15 electrically connected to each other. A small gap, through which a coolant may flow, is formed between the unit batteries 15 of each of the battery groups 14. Consequently, a coolant introduced from the coolant introduction part 12 flows through the gaps. At this time, the coolant removes heat generated by the unit batteries 15. After that, the coolant is discharged through the coolant discharge part 13 mounted at the upper part of the battery pack 11.
In the above structure, the coolant introduction part 12 and the coolant discharge part 13 are provided at the lower part and the upper part of the battery pack 11, respectively. As a result, it is necessary to provide spaces, in which coolant guide members will be mounted, at the lower part and the upper part of the battery pack 11, which further increases the overall size of the battery pack.
Meanwhile, vehicles, such as electric vehicles (EV) or hybrid electric vehicles (HEV), or power storage apparatuses may operate under various conditions. An optimal operation condition of each unit battery constituting a battery pack is generally decided within a specific temperature range although the optimal operation condition of each unit battery may vary due to various causes. Since each unit battery operates in a low temperature state in winter, on the other hand, it is necessary to adjust the battery pack such that the battery pack operates within the optimal operation temperature range. In this case, the cooling system may be stopped, or the temperature of a coolant (e.g. air) introduced into the system may be increased, such that the battery pack is not cooled but the temperature of the battery pack is increased. If the unit batteries are in a very low temperature state, however, components of the batteries may be damaged. In addition, deterioration of the battery pack may be accelerated due to abrupt increase in temperature of the battery pack.
Consequently, there is a high necessity for technology that is capable of fundamentally solving the above problems.